This invention relates to electrophotography and more particularly, to an improved overcoated electrophotographic imaging member and method of using the electrophotographic imaging member.
Electrophotographic imaging members include single or multiple layered devices comprising homogeneous or heterogeneous inorganic or organic compositions and the like. One example of a photoconductive member containing a heterogeneous composition is described in U.S. Pat. No. 3,121,006 wherein finely divided particles of a photoconductive inorganic compound are dispersed in an electrically insulating organic resin binder. Homogeneous single layer devices are well known and usually contain selenium or selenium alloys. The surfaces of selenium type photoreceptors are highly susceptible to stratches which print out in final copies.
Multiple layered photoresponsive devices comprising photogenerating layers and transport layers deposited on conductive substrates are also well known in the art and are extensively described in the patent literature, for example, in U.S. Pat. No. 4,265,990. These comprise a charge or photogenerating generating layer and a hole transport layer. Overcoated photoresponsive materials containing a hole injecting layer, a hole transport layer, a photogenerating layer and a top coating of an insulating organic resin, as described, for example, in U.S. Pat. No. 4,251,612. The disclosures of U.S. Pat. Nos. 4,265,990 and 4,251,612 are incorporated herein by reference in their entirety. Other representative patents containing layered photoresponsive devices include U.S. Pat. Nos. 4,115,116; 4,047,949 and 4,081,274. These patents relate to systems that require negative charging for hole transporting layers when the photogenerating layer is beneath the transport layer. Photogenerating layers overlying hole transport layers require positive charging but must be equal to or less than about 1 to 2 micrometers for adequate sensitivity and therefore wear away quite rapidly. While the above described electrophotographic imaging members may be suitable for their intended purposes, there continues to be a need for improved devices. For example, the imaging surface of many photoconductive members is sensitive to wear, ambient fumes, scratches and deposits which adversely affect the electrophotographic properties of the imaging member. Overcoating layers have been proposed to overcome the undesirable characteristics of uncoated photoreceptors. However, many of the overcoating layers adversely affect electrophotographic performance of an electrophotographic imaging member.
Overcoatings for photoreceptors have been disclosed in U.S. Pat. No. 4,515,882. These overcoatings comprise an insulating film forming continuous phase comprising charge transport molecules and finely divided charge injection enabling particles dispersed in the continuous phase. The imaging members have at least one photoconductive layer and the overcoating layer. Where desired, a barrier layer may be provided in the device interposed between the photoconductive layer and the overcoating layer. The devices disclosed in U.S. Pat. No. 4,515,882 can be employed in an electrophotographic imaging process in which the outer imaging surface of the overcoating layer is uniformly charged in the dark. A sufficient electric field is applied across the electrophotographic imaging member to polarize the charge injection enabling particles whereby the charge injection enabling particles inject charge carriers into the continuous phase of the overcoating layer. The charge carriers are transported to and trapped at the interface between the photoconductive layer, and opposite space charge in the overcoating layer is relaxed by charge emission from the charge injection enabling particles to the imaging surface. The overcoating layer is essentially electrically insulating prior to deposition of the uniform electrostatic charge on the imaging surface.
The mechanism by which charge passes through the overcoating to the photoreceptive surface in known devices is believed to involve the electric field, formed by corona charging of the electrophotographic device, instantly polarizing the charge injection enabling particles or species. Charge, for example, in the form of holes, is injected into the hole transport phase of the overcoating and is driven by the charging field to the interface between the overcoating and photoconductive layer. The charge is stopped at the interface by a blocking layer or because there is no injection into the photoreceptor. The negative space charge in the bulk of the overcoating is relaxed by a charge emission.
However, overcoatings such as those disclosed in U.S. Pat. No. 4,515,882 suffer from the disadvantage of relatively high light absorption and scattering in the coating due to pigment loading and particle size. Inorganic charge injection enabling particles mentioned in that patent include carbon black, molybdenum disulfide, silicon, tin oxide, antimony oxide, chromium dioxide, zinc dioxide, titanium oxide, magnesium oxide, manganese dioxide, aluminum oxides, colloidal silica, graphite, tin, aluminum, nickel, steel, silver, gold, other metals and their oxides, sulfides, halides and other salt forms, etc. Such charge injection enabling particles tend to reduce the photosensitivity of the photoreceptor. For example, one weight percent of carbon black pigment, which is a prime effective charge injection enabling species, reduces light transmission to the photosensitive layer by about 20 percent. Thus, the sensitivity of the photoreceptor is affected by absorption of some of the activating radiation absorbed by the components of the overcoating. Grinding of charge injection enabling particle to a small size for improved overcoating transparency is an extra processing step and very small particle sizes are difficult to achieve by grinding. Thus, there is a continuing need for a longer life photoreceptor having improved photosensitivity.
Although excellent toner images may be obtained with multilayered photoreceptors, it has been found that as more advanced, higher speed electrophotographic copiers, duplicators and printers are developed, photoreceptors having higher sensitivities are desired. There is also a great need for long service life photoreceptors having high photosensitivity.